first THD measurements

[Pierre]

Hello all.

Today I have started taking THD measurements of my amplifier with a simple setup: soundcard output -> amp -> dummy load -> voltage divider followed by AES-17 filter -> soundcard input

The results are not too bad, although some tweaking is required in order to get the optimum voltage level to the soundcard for every power you measure.

So far I have only measured 1KHz tones at various power levels and with a couple of load resistors.

Supply is 500VA transformer that gives (at idle) +/-50V DC with 9900uF per rail (a little bit too low), so the maximum power at 5 ohm before clipping is around 207W.

10 ohm load:
-----------------------------------
1W -> 0.023% THD
10W -> 0.033% THD
63W -> 0.17% THD
103W -> 0.5% THD

5 ohm load:
-----------------------------------
1W -> 0.037% THD
10W -> 0.07% THD
40W -> 0.19% THD
115W -> 0.2% THD
160W -> 0.51% THD
207W -> 0.65% THD

It is clear that THD increases with power level, and that at the same power output, distortion is lower with lighter load, so I suspect from the power supply, that is very weak and causes too much ripple.
Do you have any suggestions looking at this measurements?

BTW: the lower THD value obtained with a loopback cable from output to input of soundcard is 0.01% at that frequency. Should I substract that value directly from the measurements or what?

Thanks!

[Jaka Racman]

Hi,

you mentioned AES-17 filter. Did you made it yourself and if yes, cold you post a shematic diagram of it?

Best regards,

Jaka Racman

[Pierre]

Yes, but now I have checked, it is not an AES-17, but an approximation made by 3 stage RC filter, described in a IRF application note.

The values are: 470 ohm - 470nF - 680 ohm - 2.2nF - 1kohm - 1nF

Previously, a 1k / 100 ohm pot adjustable voltage divider has been connected in order to have the optimum output level for the soundcard to measure correctly for variable output power levels.

Best regards

[Jaka Racman]

Hi.

although it is not real AES-17 information is still helpful and welcome. Thanks and

best regards,

Jaka Racman

[sovadk]

The output (L C) filter could be responsible for the icressed distortion at higher load levels. The inductor is not vary linear at higher load levels, so this could be an explanation. Supply pumping chould also be it. You could make at test with different capasitors on the supply rails, to se if it makes any difference.

[Pierre]

NFB is taken (only) after output filter ;-)
I have not observed power supply pumping during all my tests even at higher power levels. But what I do have is quite a bit (perhaps 6 to 8Vpp at the max. power measured) of supply ripple due to not very large capacitors.

I will also play with dead-time, perhaps it is excessive. Now it is implemented by 6.8 ohm gate resistor in antiparallel with shottky, and with a 3k3 resistor in antiparallel with a 1n4148 at each IR2113's input, so they form a time constant with its input capacitance only during rising edge.

Perhaps I will lower that 3k3 resistors and see if distortion gets lower, but after that I will have to test the amp at full power for quite a while to be sure that it is still reliable! ;-)

Charles what do you think about these numbers? Are they too bad for a triangle-based amp?

Best regards,
Pierre

[Pierre]

Here's a small update.
I have tried with other module and reduced the dead-time resistor, noticing a very small reduction in THD. But the output ripple is no longer a nice sine wave but has peaks, I suppose some shoot-through is occuring.

I have been able to reduce the distortion measurements with a little of care while doing the tests, but what I have noticed is that THD increases with increased input frequency. I suppose that this is because my feedback network is designed to have about 31dB loop gain at up to 2KHz, then it starts decreasing. In fact, my 100Hz figures are noticeable better than what I post before (that was at 1 KHz).

Is it better to have lower feedback gain but in a wider band? Anyway, is there any other way to reduce distortion once that I have checked that dead-time is not the culprit?

Perhaps the gate resistors are too low (6.8ohm) and they are switching too fast causing more EMI problems.

If you have any ideas, they are welcome!

[IVX]

hi Pierre,
some time ago i've done THD measurement of the UcD like amp (300-1000khz switching) without any low pass filter before my old audigy..no problem up to 0.002%=audigy loopback THD. However my old subamp design (80khz) has more need for such filter, hence the internal low pass filter of the sound card quite enough for DIY THD measurement if the switching higher 300khz?

[Pierre]

I use a 3 stage RC filter just in case ;-)
I don't know if my readings are being contaminated because of not enough filtering...
My sw. freq. is around 300KHz.

Best regards,

[IVX]

Oops.. just for the logic victory, and to be alike AES17, your filter must have 4,7nF instead of 470nF!